1. Field of the Invention
This invention relates to a light source device for endoscopes which is compact in design, keeps a loss of the amount of light to a minimum, and affords low cost and small power consumption, and in particular, to a light source device for endoscopes which is used in an endoscope observation system and, even with use in an endoscope having a plurality of light guides of different numerical apertures, is capable of changing the numerical aperture of an incident light beam in accordance with the numerical apertures of the light guides.
2. Description of Related Art
Endoscopes at present are widely used in which the internal organs are observed by inserting an elongation of the endoscope in the human body, and a treatment tool inserted in a channel therefor is used when necessary, to make various treatments.
Endoscopes are roughly divided into two classes: "fiberscopes" in which visual observation is made and "electronic scopes" in which a solid-state image sensor, such as a CCD (charge coupled device), is used as image pick-up means. In particular, the latter endoscopes have been frequently used in recent years.
The illumination means of the endoscope is generally constructed with a light source device; a light guide for transmitting light supplied from the light source device to the distal end of the endoscope; and an illumination lens incorporated in the distal end of the endoscope, for spreading the light emerging from the light guide to radiate toward an object.
FIG. 1 shows an optical arrangement of a conventional light source device for endoscopes. This light source device includes a light source 1 emitting white light; filters 2 for selectively blocking rays having particular wavelengths of infrared and ultraviolet rays, of white light emitted from the light source 1 (namely, total infrared blocking filters for removing infrared rays); a condenser lens system 3 for collecting beams of light transmitted through the filters 2; and a light guide 4 for receiving the light beams collected by the condenser lens system 3 into its entrance end to transmit them to its exit end. The optical system of this light source device is designed so that the image of the bright spot of a source lamp 1a is projected on the entrance end face of the light guide 4.
In general, a xenon lamp is frequently used for the light source 1 of the light source device for endoscopes of this type, and the source lamp 1a is constructed integral with a reflecting mirror 1b. The bright spot of the source lamp 1a is condensed at the position of the focal point of the reflecting mirror 1b so that reflected light by the reflecting mirror 1b becomes nearly parallel beams.
Illumination beams transmitted through the filters 2 are collected by the condenser lens system 3, and recently, for the purpose of compactly constructing the entire light source device for the purpose of reducing the manufacturing cost, a single lens in which at least one of its lens surfaces is configured to be aspherical has been chiefly used for the condenser lens system 3. It is for this reason that, in order to efficiently condense the illumination beams from the source lamp 1a through the condenser lens system 3 on the entrance end face of the light guide 4, an aspherical lens needs to be used to correct spherical aberration and coma. In this case, the aspherical lens has a convex surface of such a shape the curvature is large in the vicinity of the optical axis of the lens and reduces progressively in separating from the optical axis.
For the illumination light source of the light source device for endoscopes, a halogen lamp, in addition to the xenon lamp, is used, and it is a common practice that the halogen lamp is constructed to be integral with an elliptical mirror. FIG. 2 shows the arrangement of such a light source device. Beams of light emitted from a halogen lamp 5a are condensed on the entrance end face of the light guide 4 by an elliptical mirror 5b, and thus the condenser lens system is dispensed with.
The xenon lamp of high luminance, however, is expensive in itself, and in general requires power consumption as high as 300 W, which is economically disadvantageous. Furthermore, a power supply for turning on the xenon lamp and a cooler are bulky. Hence, where the xenon lamp is used, the light source device for endoscopes also becomes large-sized and expensive.
On the other hand, because of the improvement of endoscope technology and the variety of application of the endoscope in recent years, the endoscope itself tends to a reduction in diameter. In keeping with this, the entrance end on the light source side of the light guide 4 also exhibits a tendency toward the reduction of diameter. In order to meet this situation, one lens surface of the condenser lens system 3 is provided with a high power to diminish the focal length as far as possible and a beam diameter on the condensing face is reduced so that a sufficient amount of light can be supplied to the light guide of reduced diameter.
However, with the developments of an endoscope apparatus and its manipulation, the following desires have recently been increased.
(1) When a 300 W xenon lamp of high luminance, which is high in cost and large in power consumption as stated above, is used in the light source device for endoscopes, it is very disadvantageous for the cost and power consumption of the device, and the device becomes large in size. Thus, a light source device for endoscopes is required which holds the brightness of illumination light usable even for the manipulation, affords low cost and small power consumption, and is compact in design. In order to fulfil these requirements, it is necessary to use an illumination lamp of low cost and small power consumption, such as a halogen lamp or a metal halide lamp.
(2) Recently, there is a growing tendency that the diameter of the insertion of the endoscope becomes small, and a light guide used in such an endoscope requires the diameter to be made smaller without decreasing the amount of transmission light. In order to satisfy this requirement, it becomes necessary to increase the numerical aperture of the light guide to improve the transmission capacity of light.
For point (1) mentioned above, however, the luminance of the halogen lamp is considerably lower than with that of the xenon lamp, and thus it is difficult for the halogen lamp to bring about illumination light of brightness required for the manipulation. Moreover, in the halogen lamp, the efficiency of conversion of an input power into visible light is as low as 10%. Hence, a sufficient amount of light cannot be obtained. Further, since the remaining 90% is converted into heat, a large-sized, expensive cooler is required. In the case of the metal halide lamp, on the other hand, since the efficiency of conversion of the input power into visible light is high, a lamp of low rated power consumption is satisfactory and does not produce heat so copiously as to cause trouble. As such, a small-sized, inexpensive cooler is used. However, the metal halide lamp is lower in luminance than the xenon lamp and is difficult to secure a fair amount of light.
As means for increasing the amount of light, it is considered that a lamp of high rated power consumption is used. In this case, however, the amount of emitted light is increased because the light emitting section of the lamp is enlarged, but that alone will not improve the luminance of the light emitting section. Thus, in the case where light is made incident from such a lamp on the light guide of small diameter, when a power supplied exceeds a certain value, little brightness is improved even though the power is made higher. In this way, an increase in rated power consumption of the lamp causes useless oversizing of the light source device and the cooler, which is unfavorable.
Consequently, in order to bring about the light source device for endoscopes which affords low cost and small power consumption and is compact in design, it is necessary that a lamp of small power consumption and low cost be used and light emitted from this lamp be made effectively incident on the light guide.
As for point (2), where the light guide of small numerical aperture which has been used in the past is used in the device in which the focal length of a condensing optical system is set so as to correspond to the light guide of larger numerical aperture, the amount of light incident on the light guide suffers a great loss.
In order to solve this problem, as set forth in Japanese Patent Preliminary Publication No. Hei 3-118509, it is known that the numerical aperture of emitted light of the condensing optical system can be changed in accordance with that of the light guide. However, that the optical system is constructed with a zoom system as the above means results in serious complication of the optical system, and causes an increase in the number of lenses and a loss of the amount of light as well. Furthermore, the entire length of the optical system is increased, and oversizing of the light source device itself is brought about.
The conventional device, as shown in FIG. 3, has met the problem in such a way that, in order to secure incident light of the numerical aperture most suitable for light guides of small and large numerical apertures, two light sources 1 are used to form separate optical paths, in which condenser lenses 3 and 3' are arranged. In this case, the use of two lamps causes oversizing of the entire light source and increases power consumption, resulting in false economy. Additionally, since the production of heat in the entire light source is increased, a large-sized cooler is required. As a consequence of this, the light source device is enlarged and becomes costly.